The present invention relates to a propeller device boosting propeller shaft torque of a marine vessel. In particular, the present invention relates to a propeller device having dual propellers, wherein the pressurized water at the outlet of the front propeller spins the rear propeller which is locked in one direction of rotation so that the torque formed on the rear propeller is transmitted on the main shaft.
A marine vessel is impelled in the sea by a propeller which is communicated with the vessel's engine from one end and with the other end of a shaft connected to a transmission. The extent of the shaft torque is substantially influential on the movement capacity of the marine vessel, since it accelerates the propeller. In essence, the output torque of the shaft is a function of several variables, such as engine power, speed and transmission reduction. The propellers are conventionally designed as to meet the torque at the highest motor speed. In principal, a higher shaft torque requires a higher engine power, which increases the cost of fuel consumption.
A conventional approach for providing a better propeller thrust is to use a dual propeller structure, wherein the propellers are coaxially mounted on the shaft. The dual propeller structure is essentially based on the principle that the torque that is received from the shaft connected with the engine is transferred to a second shaft, with which the front propeller is communicated by means of a first power transfer means (for instance a gear wheel mechanism), and that the torque that is received from the shaft connected with the motor is transferred to a third shaft that is coaxial with the second shaft, the third shaft being communicated with the rear propeller by means of the second power transfer means. An exemplary arrangement for a dual propeller structure is disclosed in U.S. Pat. No. 6,821,169. Likewise, U.S. Pat. No. 6,702,631 and U.S. Pat. No. 6,478,641 disclose a dual propeller structure.
In the dual propeller structures mentioned in the above documents, the resultant thrust slightly increases as the torque that is necessary for rotating both propellers is provided by one power source, by the shaft communicated with the engine. This is because of the fact that two separate propeller devices entail two separate sources of energy loss. Thus, each propeller device has its own power transfer means, and they cause significant mechanical losses. Moreover, the rear propeller uses the water which has already been de-energized by the front propeller, causing a loss of efficiency.